Effect of oxidation inhibitor on the low energy tribological behavior of carbon-carbon composites

In this study, the tribological performance of carbon-carbon composites impregnated with different amounts of MoSi₂ as an oxidation inhibitor were investigated. The results of the friction tests indicated that the carbon-carbon composites underwent an abrupt transition of the coefficient of friction at the frictional temperature range of 150-180°C. And the composites made with MoSi₂, exhibited lower frictional coefficient and wear rate in comparison with the composites made without MoSi₂. The composites made with 4 wt% MoSi₂ showed an improvement in activation energies for wear resistance when compared with the other composites under the present condition. These results were probably due to the consequence that the friction and wear properties of carbon-carbon composites are sensitive to the friction temperature and can be largely dependent on the adhering force between fibers and matrix-MoSi₂, the reduction of porosity, and the formation of a lubricative, powdery, debris film, formed on the friction surfaces of the carbon samples.     

Multilayer coating with self-sealing properties for carbon-carbon composites

This work proposes a simple and low cost method to deposit an effective multilayer protective coating on carbon-carbon composites (C/C). The first layer is made of molybdenum disilicide particles in a barium boron aluminosilicate glass (SABB); the second (external) layer is made of yttrium oxide modified SABB. A study of the reactions between the yttrium oxide and the SABB glass is presented. The coated C/C were submitted to thermal cycling or thermal aging tests up to 1300 °C in air. Weight losses were less than 0.5% in 50 h of thermal cycling and less then 1% in 150 h of thermal aging.     

Tensile fatigue of a laminated carbon-carbon composite at room temperature

The tensile fatigue behavior of a cross-ply carbon-carbon (C/C) laminate was examined at room temperature. Tension-tension cyclic fatigue tests were conducted under load control at a sinusoidal frequency of 10 Hz to obtain stress-fracture cycles (S-N) relationship. The fatigue limit of the C/C was found to be 213 MPa (93% of the tensile strength), and no fracture was observed at over 10⁴ cycles. The residual tensile strength of specimens that survived fatigue loading was enhanced with increase in fatigue cycles and applied stress. Observations of the fatigue-loaded specimens revealed that the formation of micro-cracks at the fiber-matrix interfaces was facilitated during fatigue loading. These interfacial cracks were concluded to protect the fibers from being damaged by matrix cracks and this behavior was considered to be the governing mechanism of strength enhancement by fatigue loading.     

Physical properties of graphite/aluminium composites produced by gas pressure infiltration method

Graphite/aluminium composites have been produced by means of gas pressure infiltration method. Two porous graphite preforms with a porosity of 10 and 13 vol%, respectively, have been infiltrated using either a commercially 99.85 pure aluminium or an AlSi7Mg alloy. Thermal expansion coefficient, electrical conductivity and flexural strength have been determined as a function of graphite preforms and metal matrices. To investigate the susceptibility of this composite system to thermal damage, specimens were thermally cycled between 60 and 300 °C up to 1020 cycles. Infiltrated graphites exhibited a significantly higher electrical conductivity (0.34-0.51 m/Ω mm²) compared to porous graphite preforms depending on graphite type and metal matrix. Thermal cycling did not influence electrical conductivity. The coefficients of thermal expansion of the composites were at least three times lower than for monolithic aluminium. Thermal cycling has reduced these values even more, most likely due to stress relaxation processes. The infiltration of porous graphite preforms with AlSi7Mg alloy or Al99.85 has increased the flexural strength of the composites resulting in values up to 105 MPa. The decrease in mechanical strength due to thermal cycling was about 10%.     

Friction performance of C/C composites prepared using rapid directional diffused chemical vapor infiltration processes

A technology used to prepare C/C composites using a rapid directional diffused (RDD) chemical vapor infiltration process has been investigated. General RDD technologies were explored, and optimal parameters were determined. The friction and wear properties of this material were researched. The results showed that in the RDD process, propylene and nitrogen were rapidly and directionally diffused into the carbon preforms enabling carbon deposition to occur from the inside of the preform to the outside. This method prevents the formation of an outer crust on the surface of preforms and facilitates uniformity of densification. With the RDD process no surface machining was required between chemical vapor infiltration (CVI) cycles thereby enabling continuous densification and reducing the CVI cycle times. The optimum processing conditions for RDD CVI were as follows; furnace temperature 950 °C; and furnace pressure 6.7 kPa. The C/C composites produced using RDD CVI processing exhibited good friction performance. Their curves of the brake moment with the velocity are stable under dry conditions, and their wet brake moment is greatly reduced. The average thickness wear is decreased to 9.5×10⁻⁴ mm/surface/stop.     

An RTM densification method of manufacturing carbon-carbon composites using Primaset PT-30 resin

This paper presents a development of carbon-carbon (C-C) composite by resin transfer molding (RTM) process. The RTM was used for both manufacturing of the resin matrix composite part as well as impregnation of the carbonized parts. Materials chosen were heat-treated T300 2-D carbon fabric and Primaset PT-30 cyanate ester. The PT-30 resin has a char yield similar to that of phenolics, very low volatiles, low viscosity at processing temperatures, and no by-products during cure, and hence, an excellent choice for RTM process. The process consists of RTM of the composite part, carbonization, RTM impregnation, and re-carbonization. The last two steps were repeated to achieve the desired density. The measured density and mechanical properties of just two times-densified C-C composite panels were superior to or nearly the same as the data in the literature by other processes. The RTM densification is about twice as fast as the resin solution method and it is environment friendly.     

Chemical vapor infiltration of C/C composites: Fast densification processes and matrix characterizations

Fast densification processes have been developed to improve the fabrication of C/C composite materials. In this work, a comparison is made between two techniques: the film boiling technique with a liquid reagent and the gas infiltration method. In both methods, the same home-made reactor was used. For the film boiling technique, the preform is either wrapped or not with a porous thermal barrier.Two different substrates have been densified, a carbon felt (RVC-2000^® from Le Carbone-Lorraine), and a 3D carbon cloth (Novoltex^® from Snecma). In situ temperature gradients and their temporal changes during the infiltration process have been recorded together with the delivered power necessary to maintain a constant deposition temperature. From these experiments, we have concluded about the following main points:

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the analysis of in situ parameters, powers and temperatures, and the associated profiles of the pyrocarbon deposits,

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the matrix quality with their associated microstructures as characterized by helium density, optical microscopy and Raman scattering experiments,

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the key role of the evolutive preforms as heat and mass exchangers during the process, and the assisted thermal fluxes inside the reactor.

This paper presents results which should allow to control automatically the process at an industrial scale.     

The effect of impregnation of activated carbon with SnCl2.2H2O on its porosity, surface composition and CO gas adsorption

Activated carbon was impregnated with different concentrations of SnCl₂.2H₂O. Unimpregnated and impregnated activated carbons were analysed by means of physical adsorption and XPS and were tested for CO gas adsorption in a PSA system. The adsorption isotherms of N₂ at 77 K were measured and showed a Type I isotherm indicating microporous carbon for all the samples. The surface area, pore volume and pore size distribution were reduced with impregnation. XPS analysis showed an increase in the intensity of Sn3d peak with impregnation. The impregnated activated carbon showed a very good adsorption ability of CO gas compared to the unimpregnated sample. The adsorptive species responsible for CO gas adsorption was confirmed to be SnO₂ instead of SnO due to the former’s comparative thermodynamic stability.     

Manufacturing 2D carbon-fiber-reinforced SiC matrix composites by slurry infiltration and PIP process

Sub-micrometer SiC particles were firstly added to the preceramic solution in the first infiltration step to enhance the mechanical properties of 2D C_f/SiC composites fabricated via polymer infiltration and pyrolysis (PIP) process. The effects of pyrolysis temperature and SiC-filler content on microstructures and properties of the composites were systematically studied. The results show that the failure stress and fracture toughness increased with the increase of pyrolysis temperature. SiC filler of sub-micron scale infiltrated into the composites increased the mechanical properties. As a result, for the finally fabricated composite infiltrated with a slurry containing 40 wt.% SiC filler, the failure stress was doubled compared to that without SiC filler addition, and the fracture toughness reached ≈10 MPa m^1/2.     

Inhibition of catalytic oxidation of carbon/carbon composites by boron-doping

The inhibition effect of high temperature boron-doping on the catalytic oxidation of carbon/carbon composites was investigated. Boron-doping at 2500 °C was found to improve the oxidation resistance of catalyst-loaded composites. Evident inhibition mechanisms include the reduction of active site number by increasing the crystallite size and the site blockage by formed boron oxide. Boron-doping at less than 1.0 wt.% was found to almost completely suppress the catalytic effect of calcium acetate after a slight carbon conversion. This inhibition effect was much less significant in the case of potassium-catalyzed oxidation where only a slight inhibition effect was observed. This is believed to be the essential result of the unique properties of potassium catalyst. Due to its wetting ability and mobility, potassium catalyst could form and maintain good interfacial contact with any exposed carbon surface regions.     

Injection and stabilization of mesophase pitch in the fabrication of carbon-carbon composites: Part II. Stabilization process

In the fabrication of carbon-carbon composites by mesophase injection through a fiber preform, it is essential to stabilize the flow-induced microstructure in the flow channels and to prevent relaxation and exudation of the mesophase. Oxidation stabilization studies were conducted on preforms injected with the naphthalene-based AR mesophase pitch. Oxidation mass gain (OMG) curves at 170, 222, and 270 °C were generated for 60°-wedges cut from full size composite disks. The rates of OMG at 170 °C of first- and second-cycle injection wedges and full-size disks were comparable to those using as-spun filaments 30 μm in diameter, and particles sieved to 200 to 340 μm. The results suggest that oxygen is accessible deep into a mesophase matrix and the transport is facilitated by connected array of shrinkage cracks. Oxidation at 170 °C has strong advantage over higher oxidation temperatures by having a higher carbon yield and lower OMG threshold and thus oxidation time required for stabilization. The 60°-wedges could be stabilized at 170 °C after a 25 h oxidation with a 7.2% OMG and attaining a carbon yield above 85%.     

The characterization of nitrogen-enriched activated carbons by IR, XPS and LSER methods

The aim of this work is to study the efficiency of the nitrogen enrichment by urea of lignites and the induced changes of the adsorptive properties towards volatile organic compounds (VOCs) of the activated carbons derived from these modified precursors. The study is made using infrared and X-ray photoelectrons spectroscopies and the LSER (linear solvation energy relationship) modeling. Four activated lignites derived from the same raw material, original or enriched with nitrogen, are characterized in this way. The effect of the chemical treatment by urea and of the burn-off amount are investigated in term of evolution of the chemistry of the studied materials. The influence of these parameters on the selective behavior of the activated lignites towards two pairs of VOCs is also discussed in terms of molecular interactions using the LSER approach. The results show that the chemical treatment of the raw material is successful, leading to significant enrichment with nitrogen under pyridinic form at the surface of the activated carbons. Moreover, they reveal some selective properties well explained by the LSER analysis and spectroscopic measurements. The selective character of the studied materials is modulated by the duration of the activation step.     

Influence of porosity and total surface area on the oxidation resistance of C/C composites

The oxidation tests of five different porosity C/C composites for non-isothermal oxidation and three different porosity samples for isothermal oxidation in O₂ and two samples which had approximate porosity and different total surface area values for static air oxidation were performed. Results show that the oxidation was controlled by the total surface area other than by the open porosity or the densification degree of samples or amount of pore entrance. The oxidation occurred preferentially at the fiber-matrix interface and pyrocarbon-pyrocarbon interface, and then progressed along the micro-cracks between the interfaces.     

Carbon/carbon-boron nitride composites with improved wear resistance compared to carbon/carbon

This paper describes the fabrication of a carbon fiber reinforced/carbon-boron nitride (C/C-BN) hybrid matrix composite for possible use in aircraft brakes. These composites were fabricated via liquid infiltration of a liquid crystalline borazine oligomer into a low-density carbon fiber/carbon matrix (C/C) composite. The friction and wear properties of the C/C-BN were explored over the entire energy spectrum for aircraft braking using an inertial brake dynamometer. The C/C-BN composites with densities of 1.55 g/cc displayed wear rates 50% lower than values observed with C/C samples with densities of approximately 1.75-1.8 g/cc. This includes the near elimination of wear from 300 to 600 kJ/kg, which represents the normal landing regime for aircraft brakes. This encouraging behavior is attributed in part to the improved oxidation resistance of the BN at high energy levels and the ability of the BN to facilitate formation of a stable wear film at lower energy levels. The coefficient of friction, while being slightly lower than the values for C/C, appeared much less sensitive to changes in energy level.     

Characteristics of carbon films prepared by plasma-based ion implantation

A series of carbon films have been prepared by plasma-based ion implantation (PBII) with C on pure Al and Si. Emphasis has been placed on the effect of implanting voltage on the characteristics of these films. The structures of the films were analyzed by X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. The morphologies were observed by atomic force microscope (AFM). Surface hardness and electrical resistivity were also measured. The results indicate that the characteristics of these films are strongly dependent on the implanting voltage. An implanting voltage threshold value ranging from 3 to 5 kV starts to form a C-substrate transition layer owing to C⁺ ions implanted into the substrate. The transition layer exhibits a gradual change in composition and structure and effectively connects the carbon film and the substrate. Also, an implanting voltage threshold value ranging from 5 to 10 kV starts to form diamond-like carbon (DLC) films. An increasing voltage causes the resultant DLC films to be smoother and more compact. Moreover, Raman spectrum, chemical state of C1s, surface hardness and electrical resistivity all prove an optimum voltage of approximately 30 kV corresponding to the lowest ratio of sp²/sp³.     

Inhibition of catalytic oxidation of carbon/carbon composites by phosphorus

The inhibition effectiveness of thermally deposited phosphorus (P) compounds on the carbon oxidation catalyzed by potassium or calcium acetate has been investigated. The P deposit was formed by impregnating carbon/carbon composite samples with methanol solution of methyl-phosphoric acid or phosphorus oxychloride and heating at ca. 600 °C. An amorphous layer formed by a relatively large amount of P deposit functioned as a barrier for the access of the catalyst to the carbon surface even though it had almost no barrier effect for O₂ access. The catalytic effect of calcium was almost completely suppressed by such deposit, but the effect of potassium was only partially suppressed due to the superior wetting ability and mobility of potassium species. Small amounts of P deposit showed similar inhibition effects on non-catalyzed oxidation, while their effects on catalytic oxidation were not as good. Characterization of P-deposited carbon samples by XPS, XRD, SEM and TPD, as well as ab initio MO calculations, suggested that the inhibition effect mainly resulted from the formation of oxygen-containing P groups which may include metaphosphates, C-O-PO₃ groups and C-PO₃ groups. Those groups are suggested to act as a physical barrier against carbon/catalyst interfacial contact as well as to block the active carbon sites. The presence of bridge oxygen bonded to a carbon site and a P group appears to be a critical factor for maintaining the inhibition effect. Indeed, the loss of such oxygen or connecting bond seems to result in loss of inhibition.     

Infrared radiation properties of the carbon-carbon composite and their application to nondestructive detection of its defects

Infrared radiation properties and surface characteristics of C/C composites and graphites were examined at temperatures in the range of 293-373 K from the viewpoint of the nondestructive detection of defects in these materials. The radiation temperature of specimen surface and its variation were quantitatively evaluated on the basis of true specimen temperature, ambient temperature, emissivity and radiosity coefficient to obtain data applicable to the thermographic detection of defects. It was found that the larger the pore size and roughness of specimen, the larger the variation of data points. Graphite specimens with different artificial flaws 1-10 mm in diameter and 1-8 mm in depth were examined by thermography, and the minimum difference in radiation temperature at a defect to be detected was obtained with regard to the flaw size.     

Injection and stabilization of mesophase pitch in the fabrication of carbon-carbon composites. Part I. Injection process

The fabrication of carbon-carbon composites by injection of low viscosity mesophase pitch through a fiber preform followed by stabilization and carbonization was examined. The fully transformed mesophase MOMP and AR pitches were injected through either soft or rigidized disk preforms 35 mm thick and 68 mm in diameter. Injection provided good even filling of major flow channels and fiber bundles. Flow-induced fibrous microstructures were retained by quenching and preserved by stabilization upon carbonization. A second injection cycle was effective in filling voidage created by thermal densification. A third cycle was applied, but required severe injection conditions and provided only incremental improvement. The carbon-carbon composite reached a density of 1.8 g/cm³ after three injection cycles.     

Microstructure of wood charcoal prepared by flash heating

Carbonized wood prepared by flash heating at 800 °C for 1 h shows a different microstructure and surface chemical structure than char formed after slow heating at 4 °C/min to 800 °C for 1 h. Flash heating produces pores that are surrounded by aggregates of carbon structures 25 to 100 nm in cross section. The carbon structures are built up of clearly visible graphene layers that are often curved and overlap each other in a disordered manner. The layers consist of a considerable number of oxygen-containing functional groups. The results suggest that the formation mechanism of the microstructure in wood carbonized by the flash heating process seems to originate from fragmented and oxygen-containing pyrolysis compounds in contrast to conventional heating.